In above code Firefox uses the property opacity:x for transparency, where IE uses filter:alpha(opacity=x).
For Firefox x takes value from 0.0 - 1.0. A lower value makes the element more transparent.
For IE x can take value from 0 - 100. A lower value makes the element more transparent.

Note:
1. Change the image url which is show in red color with your own image url.
2. Opacity value of zero will make image invisible.